Is gum trimming normal after root canal?

It is highly probable, based on what you wrote that there was gum tissue overgrowth around the temporary tooth between visits.  Your dentist, if the word trimming was used, should not confuse you with gum surgery.  It is a minor procedure.  However make sure you are satisfied with the HEALED appearance and that the margins of the new crown is not visible  (otherwise a new post-healing impression and new crown may also be indicated).  Hope this answers your concern.

Unfortunately without photos and X-rays it is very difficult to give you an opinion of what needs to be done.  It is not unusual to have some gum surgery to improve the restorative result in situations like you are facing.  My suggestion is to have a conversation about this with your dentist to get some clarity about why the procedure is being recommended.  I'm sure they will help you understand things better.  Good luck!